In what most had billed as a routine win, Caroline Wozniacki slipped to a straight sets defeat against world number nine Samantha Stosur.
A win would have guaranteed Wozniacki the world number one spot at the end of the season, but she was outplayed by the in-form Stosur and lost 6-4 6-3.
Wozniacki has now slipped to second favourite to win the WTA Championship at 12/5, behind Kim Clijsters at 2/1.
The Dane should still progress to the semis as she’s heavily backed to beat Francesca Schiavone at 1/3 with the French Open champion back at 10/3 to win.
Schiavone has the upper hand on Wozniacki in the head-to-head record between the pair, having won two out of the three meetings between them.
However, it was the Dane who triumphed last time out when she beat Schiavone in straight sets in Montreal earlier this year.
Stosur’s win has led to bookmakers slashing odds on her both winning against Elena Dementiava and the competition as a whole.
Dementiava was comfortably beaten by Wozniacki in her opening game and is only made 9/4 to beat Stosur, while the Australian is 1/3 to win.
The head-to-head record between the two is pretty even, with Dementiava having won four and Stosur three of their seven meetings.
Last time out in the US Open, Stosur won through in a tight game, only decided by a third set tie-break.
In fact the last three meetings between these two have gone to three sets with Stosur winning the first set each time, so a bet of 8/15 on Stosur winning the first set and the match seems a very good one.
If the Australian does beat Dementiava, it will set her up nicely for a likely semi-final against the loser of the crunch game between Kim Clijsters and Vera Zvonareva.
Stosur is now rated 4/1 to win the trophy, down from 8/1 before her win over Wozniacki.
Wozniacki is 5/1 to lose the first set and win the match. Follow the latest WTA news at Ladbrokes.com.